There are two switches that can be activated by two different players at the beginning of The Package -- this Easter Egg is different from the two panels that open Dr. Halsey's lab at the end of the level. You can do this on Normal; no need to start a Legendary game. Before you begin your switch hunting, eliminate all the enemies in the first area. Try to grab a Ghost as well. At the southern end of the lake is a shack. On the north side of the shack, look for a large crate against the wall. Use the Ghost or a melee attack to move this crate.

One person should stay right here. The other person should continue south and get in the Scorpion tank. Use it to destroy the sniper roosts and the turrets just to the south. Now, walk off the edge of the short cliff due south of where you picked up the Scorpion. Turn east and look for a glowing green panel on the cliff side where its meets the water. As soon as you hit this switch, a second switch will appear only very briefly on the wall in the lake, so coordinate this with your partner. When the second switch is activated in the lake, four Banshees will appear on the roof of the nearby building to the north.

Climb up and hop into a Banshee. You can freely explore the massive level, even the dogfight to the south. If you take out the AA cannons BEFORE you hit the switches (the AA guns seem to become invincible if you get the Banshees first) with the Scorpion, you can fly the Banshees through Sword Base's entrance gate and explore even more of the level.

You can even land on the mountains all around the area and get out without dying. Enjoy!